So called “leather-like” sheets such as synthetic leather, artificial leather or the like which comprise ultra-fine fibers and a polymer elastomer has excellent characteristics which are not shown in natural leathers, and widely used in various applications. For producing such a synthetic leather or an artificial leather, wet coagulation methods in which after impregnating a fibrous sheet-like product with a solution of polymer elastomer such as polyurethane and then said fibrous sheet-like product is immersed in water or an aqueous organic solvent to wet-coagulate the polymer elastomer, is generally applied (for example, JP-A-2000-336581).
However, in order to achieve a high strength or a dimensional stability, a much amount of polyurethane is used, and, therefore, leather-like sheets are expensive due to material cost of polyurethane and complexity of production process, etc. Furthermore, organic solvents compatible with water such as N,N′-dimethylformamide are used in the process of impregnating with polyurethane, but these organic solvents are not preferable in view of working circumstance.
In addition, it is pointed out that, when the polymer elastomer becomes rich, the hand of leather-like sheet may become rubber like, and dense feeling similar to natural leathers may not be obtained. Furthermore, it is not preferable to contain a polymer elastomer, not only in view of the hand, but also in view of recyclability of which much is made for the purpose of protecting environment and resources. For example, for recycling methods of polymers, a recycling method of polyesters by decomposition or a recycling method of polyurethane by decomposition is being investigated, but these methods are mainly applied to polymers of a single component. As stated above, in a composite material in which a fiber and a polymer elastomer are integrated, it becomes difficult to apply those methods because the methods of pyrolysis of the respective polymers are different. Accordingly, it becomes necessary to separate the fiber and the polymer elastomer, but it is generally very costly and it is also impossible to separate those perfectly.
Other than that, a yellowing problem of polyurethanes or the like with NOx gas is pointed out, and there remain various problems such that it is difficult to obtain a white suede-like sheet.
In view of these, a leather-like sheet in which polymer elastomer such as polyurethane is decreased or they are substantially not contained, is desired.
However, as a result of our efforts to obtain a leather-like sheet which substantially does not contain a polymer elastomer, we found that it is difficult to obtain a leather-like sheet with a hand having a repulsive feeling similar to that which comprises a polymer elastomer. This means that it is difficult to satisfy various consumers' needs, and it is desired to solve this problem.
However, with respect to a leather-like sheet which does not contain a polymer elastomer, almost no investigation has been made, and it is the present situation that means for solving this problem has not been found.